1. Field of the Invention
This invention relates to inhibiting electrical breakdown by water treeing in dielectric materials which are based on ethylene polymers.
2. Description of the Prior Art
Compositions based on polyolefins are well-known and they are used extensively as insulation materials for wire and cable. As insulation materials, properties of the composition such as intrinsic electrical strength, corona resistance and resistance to treeing are important.
Intrinsic electrical breakdown is the catastrophic failure of a dielectric accompanied by arcing or discharge through an ionized channel in the dielectric. The intrinsic dielectric strength is considered to be an inherent property of the dielectric material.
In power cable applications for transmitting relatively high voltage loads such as, 5KV and above, corona may be a problem because it may lead to the premature breakdown of the cable insulation. Corona is an electrical plasma resulting from the ionization of a gaseous dielectric in regions of high electrical field. Corona resistance is the ability of a dielectric to withstand the corrosive action of an electrical plasma in contact with it.
When used as a high voltage power cable insulation, olefin base compositions undergo a prebreakdown phenomenon known as treeing. This type of damage progresses through a dielectric section under electrical stress so that, if visible, its path looks something like a tree. Treeing can occur and progress slowly by periodic partial discharge, it may occur slowly in the presence of moisture without any partial discharge, or it may occur rapidly as the result of an impulse voltage. Trees may form at sites of high electrical stress such as contaminants or voids in the body of the insulation or at irregularities at the insulation-semiconductive screen interface.
In solid organic dielectrics, treeing is the most likely mechanism of electrical failures which do not occur catastrophically, but rather appear to be the result of a more lengthy process. It is desired to extend the service life of olefin-insulated cables by modification of the insulating materials so that trees are initiated at higher voltages than usual or so that the rate of growth of trees is reduced once initiated.
Electrical treeing results from internal electrical discharges which decompose the dielectric. Although high voltage impulses can produce electrical trees, and the presence of internal voids and contaminants is undesirable, the damage which results from application of moderate a.c. voltages to electrode/insulation interfaces which contain imperfections is more commercially significant. In this case, very high, localized stress gradients can exist and with sufficient time lead to initiation and growth of trees which may be followed by breakdown. An example of this is a high voltage power cable or connector with a rough interface between the conductor or conductor shield and the primary insulator. The failure mechanism involves actual breakdown of the modular structure of the dielectric material perhaps by electron bombardment. Much of the prior art is concerned with the inhibition of electrical trees.
Water treeing is a deterioration of a solid dielectric material which is simultaneously exposed to moisture and an electric field. It is a significant factor in determining the useful life of buried high voltage power cables. Water trees initiate from sites of high electrical stress such as rough interfaces, protruding conductive points, voids, or imbedded contaminants but at a lower field than that required for electrical trees. In contrast to electrical trees, water trees are characterized by: (a) the presence of water is essential for their growth; (b) no partial discharge is normally detected during their growth; (c) they can grow for years before reaching a size where they may contribute to a breakdown; (d) although slow growing, they are initiated and grow in much lower electrical fields than those required for the development of electrical trees.
Thus, intrinsic electric breakdown, failure by corona, electrical treeing and water treeing are different and the mechanisms for each are different. It follows that a different solution is required to effect an improvement in a dielectric material for each mode of failure involved.
Additionally, it is known that when cross-linked olefin polymers, particularly polyethylene, are used for power cable insulation, a crosslinking agent may function as a water treeing inhibitor. When dicumyl peroxide is used as the crosslinking agent in polyethylene, for example, the peroxide residue functions as a tree inhibitor for some time after curing. However, these residues are eventually lost at the temperatures of cable service. Therefore, in order to be an effective water treeing inhibitor an additive must be such that it is retained in the olefin composition at the temperature of cable service.